An Optical Transport Network (OTN) is defined by ITU-T as a set of optical elements that are connected by optical links and that are capable of providing transporting, multiplexing, switching, management, supervision functionality, and survivability of optical channels carrying client signals.
An OTN uses digital wrapper technology that provides a method for encapsulating data in an optical data unit. Generally, digital wrapping involves grouping a number of existing optical data units together into one entity that can be more efficiently managed with a small allocation of overhead and forward error correction (FEC) bytes. There are multiple levels of hierarchy for encapsulation, including an Optical channel Payload Unit (OPU) for encapsulation of client data, an Optical channel Data Unit (ODU) for encapsulating the OPU data, an Optical channel Transport Unit (OTU) for encapsulating the ODU data.